Two-Dimensional Quasi-Steady and Impulsive Reconnection: A Comparative Study Using Particle-in-Cell and Hall MHD Simulations

We present two 2D studies of collisionless magnetic reconnection dynamics obtained from our new fully electromagnetic PIC code (ExPIC), and compare them with results obtained earlier from Hall MHD simulations using the same initial conditions. Our studies include realistic values of me/mi. The first study involves the scaling of the maximum electron outflow velocity from the reconnection region in the GEM challenge problem, which, according to Hall MHD models, scales as the electron Alfven speed. Our PIC simulations show flows that are uniformly smaller than the electron Alfven speed, with deviations that increase in magnitude as the mass ratio reaches its actual physical value. The second study involves forced magnetic reconnection in a plasma sheet driven continuously by inward boundary flows. It is observed in the PIC simulations that the reconnection rate in the linear regime increases algebraically in time, and is followed by a sudden impulsive enhancement in the nonlinear regime, qualitatively similar to that seen in Ma and Bhattacharjee's earlier Hall MHD simulation (1996). However, the current sheet produced is more singular and the impulsiveness greater in the Hall MHD simulation than it is in the PIC simulation. Quantitative comparisons between PIC and Hall MHD simulation results will be given, and kinetic mechanisms that produce differences between Hall MHD and PIC models will be discussed.